This project aims to determine physiologically critical functions of NF-kappaB proteins and their regulators in the context of specific biologic reactions. Regulators include the inhibitory IkappaB proteins, as well as proximal activators. Research is based on the discovery of specific defects (especially of the immune system) in mice rendered deficient for various NF- kappaB proteins or their regulators. The ultimate goal is to identify critical molecular targets of the NF-kappaB factors in specific immune responses and to identify the essential signals that activate the factors. Previously we have generated mice deficient in both the p50 and the p52 subunits of NF-kappaB, two highly homologous proteins likely to encode redundant functions. The double knockout mutant mice are profoundly impaired in their immune system, with many defects not seen in either single knockout, consistent with redundant activities of these two proteins. Significantly, only the double knockout mice are blocked in the development of mature osteoclasts and of mature B cells. We further determined in adoptive transfer experiments that the block in generation of a mature B cell population is both intrinsic to B cells as well as cell autonomous. The loss of these proteins partially impairs formation of Immature B cells in the bone marrow, a defect that is recapitulated in in vitro bone marrow cultures. Those immature cells that are generated in the bone marrow still migrate to the spleen, however, once there, these B cells are no longer able to undergo final maturation. The result is a complete absence of longer-lived recirculating mature B cells, primarily because of premature death of these cells. These results provide compelling evidence that NF-kB p50 and p52 proteins are critical for cell survival during development. Finally, we have discovered that osteoclast progenitors do form in the absence of both p50 and p52, but that these cells no longer respond properly to the RANK ligand, and thus fail to develop into osteoclasts.